Electrostatic spraying devices, in which an electric charge is imparted to a liquid before or after it is forced through a nozzle, to provide small, electrostatically charged droplets, are widely known and used for a variety of purposes, including spraying of paint and chemicals. These may be distinguished from electrohydrodynamic (EHD) devices, in which an electric charge of sufficient intensity is applied to the fluid to induce aerosolization.
Coffee, U.S. Pat. No. 4,962,885, for example, describes an apparatus that comprises a conductive nozzle charged to a potential of up to 20,000 volts closely adjacent a grounded electrode. The electric field generated between the nozzle and the grounded electrode is sufficiently intense to atomize liquid delivered to the nozzle into finely charged particles. Dvorsky et al., U.S. Pat. No. 6,302,331 describes a system in which fluid is delivered to a nozzle that is maintained at high electric potential relative to a proximate electrode to cause aerosolization of the fluid, with the fluid emerging from the nozzle in the form of a so-called Taylor cone (as described, for example, in M. Cloupeau and B. Prunet-Foch, “Electrohydrodynamic Spraying Functioning Modes: A Critical Review,” J Aerosol Sci., Vol. 25, No. 6, pp. 1021, 1025-1026 (1994)). The Taylor cone shape of the dispensed aerosolized fluid results from a balance of the forces of electric charge on the fluid and the fluids own surface tension. Desirably, the charge on the fluid overcomes the surface tension at the tip of the Taylor cone, so that a thin jet of fluid forms, which separates a short distance from tip into a fine aerosol having uniform droplet size.
More recently, there has been a recognition that EHD devices are useful for producing and delivering aerosols of therapeutic products for inhalation by patients. Inhalation therapy for delivering both locally and systemically active drug compounds is increasing as the health-care community recognizes the benefits this route offers to patients. EHD aerosol delivery systems are expected to revolutionize inhalation therapy. These systems are more efficient and reproducible than existing inhalation devices. EHD devices can deliver a soft (isokinetic) cloud of uniformly sized particles directly to the lungs with better than 90 percent efficiency, and without the need for liquid propellants or other pressurized systems. The aerosol is delivered using the patient's own breath (inspiration), whereby the patient can easily achieve the drug delivery at normal inhalation rates. The delivery mechanism is especially suited to use with infants, young children, seniors, and patients with an impaired respiratory function.
Zimlich, Jr., et al., U.S. Pat. No. 6,397,838 discloses a pulmonary aerosol delivery device that delivers an aerosolized liquid cloud having therapeutic properties to a user's lungs. The compact and convenient device includes a housing of such size that it can be held in a user's one hand with an exit opening in the housing for directing the aerosol to the user's mouth. The aerosolizing apparatus (i.e., EHD nozzle) includes a plurality of spray sites (i.e., tip ends) that cooperate with discharge electrodes and reference electrodes downstream respectively of the tip ends to result in an aerosolized spray from at least one tip end. The multiple spray site nozzle can achieve larger dosages.
While U.S. Pat. No. 6,397,838 presents a significant advance over previously known aerosol delivery devices, opportunities exist for improvement. For instance, in the device described in the '838 patent, the EHD nozzle is to be pointed downwardly in order for each nozzle tip to dispense consistently. However, most users prefer to be upright when using the dispenser. Consequently, the dispensed aerosolized liquid had to be directed through a bend to the exit opening. Momentum of the aerosolized droplets tends to deposit some of liquid onto the exit opening, reducing the effective dose delivered to the user. In addition, wetting of the interior of the EHD nozzle itself may degrade performance. Since most, if not all, liquids dispensed by pulmonary delivery devices to some extent are conductive, wetting tends to dissipate the desired electric fields within the EHD nozzle, especially if a conduction path should form between the discharge and reference electrodes.
Atterbury et al., published U.S. application no. US 2004/0195403 A1 provides some solutions to the wetting of the interior of the nozzle through shielding of discharge electrodes. Branching channels formed in the spray nozzle provide a controlled pressure drop to a plurality of circumferentially arranged nozzle tips. The controlled pressure drop to each nozzle tip advantageously allows increased dosage production with multiple tips while avoiding undesired variations in the flow rate seen at each nozzle tip, which would affect the achieved particle size. One or more dissociated discharge electrodes, shielded from the spray nozzle, may be positioned upstream or downstream of the plane of the nozzle tip of the spray nozzle, which preferably neutralizes the charge applied to the atomized droplets. It is desirable to have an EHD nozzle that produces a completely electrically neutralized aerosolized liquid, since droplets that retain a charge tend to compound wetting problems and may also limit the therapeutic effect of the inhaler.
We have discovered, however, that the system described by Atterbury et al. can be improved upon. In particular, in embodiments where the discharge electrode is upstream of the nozzle, there is a tendency for the insulating shield located between the nozzle and the discharge electrode to be wetted, as droplets of atomized liquid are “pulled” by the nature of the electric field back towards the discharge electrode. Consequently, a need exists for an improved EHD nozzle that provides more efficient delivery of a consistent dose of aerosolized particles, particularly one that is suitable for use in a portable pulmonary aerosol delivery device.